Among disks that may constitute a redundant array of independent disks RAID, several kinds of disks have poor performance but a large capacity, such as NL-SAS disk. As a result, when a RAID including NL-SAS disks is degraded, rebuilding for a new disk will last for a rather long time, or even days. In this situation, the RAID stays in a risky state for a too long term (i.e., potential data unavailability/data loss), because any other disk failure from this RAID will cause damage of the entire RAID (in the case of RAID-5 type). All data from this RAID will become unavailable. In a word, when the RAID is in a degraded state, clients will frequently encounter a data unavailable risk.
However, RAID has been rebuilt for a long term since failure of a first disk, and data in some RAID stripes have been recovered. Therefore, in case that one disk in the RAID has been rebuilt for a while and then a further disk is damaged, there still exists a possibility of recovering some data.